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A good rule of thumb is, one heat range colder for every 75–100hp added.

Colder is better especially with high compression, but if you use too cold of a plug it equates to less power as well (incomplete fuel burn) So running too cold, in general usually leads to plug fouling and not much else.

From NGK:
The spark plug design determines its ability to remove heat from the combustion chamber. The primary method used to do this is by altering the internal length of the core nose. In addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges.

*When a spark plug is referred to as a “cold plug”, it is one that transfers heat rapidly from the firing tip into the engine head, keeping the firing tip cooler.

*A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter.
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